Doctors are reporting an unusual phenomenon in COVID-19, where patients have critically low levels of blood oxygen, but no breathlessness. This has been described as ‘silent hypoxia’ or ‘hypoxaemia’ – meaning low blood oxygen.

The lack of breathing difficulties means low oxygen levels may not be spotted until the patient is seriously ill. People hospitalised with COVID-19 are likely to have oxygen levels monitored, and there is increasing access to hospital grade pulse oximeters outside of hospital. However, it is important that any reassurance offered by these tests does not delay or prevent appropriate medical attention.

Hypoxaemia usually occurs when damage and inflammation to the air sacs in the lungs mean they are not able to transfer oxygen into the small blood capillaries. This makes the person breathe faster to try and get more oxygen. However, while many people with COVID-19 report breathing difficulties, others do not.

Many people with breathlessness and suspected COVID-19 will have their blood oxygen measured with a pulse oximeter out of hospital. Rest homes, nursing homes and ambulance services usually stock these devices. However, there is no evidence to say that smart phone apps or fitness trackers are accurate enough for this purpose.

Where did the story come from?

The phenomenon of silent hypoxia or hypoxaemia was first reported in studies from China, although the studies did not indicate how common this was. It has since been noticed by doctors in the UK and other European countries.

What is the basis for the claim?

A letter published by doctors from China discussed the characteristics and treatment of 168 patients who died of COVID-19 between 21st and 30th January 2020 across 21 hospitals in Wuhan. Their average age was 70 years and 75% had underlying health problems. All 168 were reported to have received oxygen while in hospital but:

  • 27% received oxygen via face mask or nasal cannula only
  • 43% received non-invasive positive pressure ventilation
  • 20% were intubated and received mechanical ventilation

The report said this could be because of insufficient ventilators, or because ‘some patients with severe hypoxemia did not have other symptoms, such as shortness of breath, also called silent hypoxemia.’

Another editorial in the journal Intensive Care Medicine described how COVID-19 pneumonia is ‘a specific disease’ where there can be severe hypoxemia associated with near normal respiratory compliance (the ability of the lungs to expand and take in air) – something not normally seen in acute respiratory distress syndromes. Such patients may be breathing quite normally.

A letter to the British Journal of Anaesthesia draws lessons from aviation medicine and suggests that the syndrome of silent hypoxaemia could be because carbon dioxide is low as well as oxygen – when it is high blood carbon dioxide that usually causes breathlessness.

Some media have suggested that people with COVID-19 symptoms should monitor their oxygen levels at home and suggest purchasing a pulse oximeter. Although FitBits and smartphones can monitor oxygen levels, a review has shown that these monitors on smart devices are not accurate enough to be used for this purpose.

What do trusted sources say?

The WHO advises that ‘if you have minor symptoms, such as only a slight cough or a mild fever, ‘there is generally no need to seek medical care’. They advise such people to stay at home, monitor their symptoms, and follow national guidance on self-isolation.

The WHO advises that people get medical attention immediately if they have any breathing difficulties or pain or pressure in the chest.

Analysis by EIU Healthcare, supported by Reckitt Benckiser



  1. Xie J, Tong Z, Guan X, Du B, Qiu H. Clinical Characteristics of Patients Who Died of Coronavirus Disease 2019 in China. JAMA Netw Open. 2020;3(4):e205619. doi:10.1001/jamanetworkope

Reading list

  1. Gattinoni, L., Chiumello, D., Caironi, P. et al. COVID-19 pneumonia: different respiratory treatments for different phenotypes?. Intensive Care Med (2020).
  2. Ottestadt W et al. COVID-19 patients with respiratory failure: what can we learn from aviation medicine? British Journal of Anaesthesia (correspondence). Published: April 18, 2020 DOI: